Biaxial orientation of polyolefins, particularly polypropylenes, produces films which have applications in the polyolefin film business, for example snack food packaging, cigarette overwrap, electronic components wrapping, packaging tape, and shrink film. The polymers normally employed in the preparation of biaxially oriented films are isotactic homopolymers with high stereoregularity, although on some occasions the use of syndiotactic polymers has been proposed. Also suitable are co-polymers of isotactic polypropylenes with a small content of ethylene (mini-random co-polymers).
Isotactic polypropylene is one of a number of crystalline polymers which can be characterized in terms of the stereoregularity of the polymer chain. Various stereo specific structural relationships denominated primarily in terms of syndiotacticity and isotacticity may be involved in the formation of stereoregular polymers of various monomers.
Isotactic polypropylene is conventionally used in the production of relatively thin films in which the polypropylene is heated and then extruded through dies and subject to biaxial orientation by stressing the film in both a longitudinal direction (referred to as the machine direction) and lateral direction sometimes referred to as the tenter direction. The structure of isotactic polypropylene is characterized in terms of the methyl group attached to the tertiary carbon atoms of the successive propylene monomer units lying on the same side of the main chain of the polymer. That is, the methyl groups are characterized as being all above or below the polymer chain. Isotactic polypropylene can be illustrated by the following structural formula: ##STR1##
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Stereoregular polymers, such as isotactic and syndiotactic polypropylene can be characterized in terms of the Fisher projection formula. Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each "m" representing a "meso" dyad, or successive methyl groups on the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic propylene polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Syndiotactic polypropylene can be illustrated by the following structural formula: ##STR2##
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Syndiotactic polymers are semi-crystalline and, like the isotactic polymers, are insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer, which is non-crystalline and highly soluble in xylene. An atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product. In most cases, the preferred polymer configuration will be a predominantly isotactic or syndiotactic polymer with very little atactic polymer.
The isotactic polymers normally employed are typically prepared through the use of conventional Ziegler-Natta catalysts of the type disclosed, for example, in U.S. Pat. Nos. 4,298,718 and 4,544,717, both to Myer et al. U.S. Pat. No. 5,573,723 to Peiffer discloses a process for producing biaxially-oriented polypropylene film based on an isotactic polypropylene homopolymer or propylene ethylene co-polymers. Other co-polymers of propylene and alpha-olefins having from 4-8 carbon atoms also may be employed in the Peiffer process.
Catalysts employed in the polymerization of alpha-olefins may be characterized as supported catalysts or unsupported catalysts, sometimes referred to as homogeneous catalysts. Traditional supported catalysts are the so-called "conventional" Ziegler-Natta catalysts, such as titanium tetrachloride supported on an active magnesium dichloride as disclosed, for example, in the aforementioned patents to Myer et al. A supported catalyst component, as disclosed in the Myer '718 patent, includes titanium tetrachloride supported on an "active" anhydrous magnesium dihalide, such as magnesium dichloride or magnesium dibromide. The supported catalyst component in Myer '718 is employed in conjunction with a co-catalyst such and an alkylaluminum compound, for example, triethylaluminum (TSA1). The Myer '717 patent discloses a similar compound which may also incorporate an electron donor compound which may take the form of various amines, phosphenes, esters, aldehydes, and alcohols. Metallocene catalysts are often employed as unsupported or homogeneous catalysts, although, as described below, they also may be employed in supported catalyst components.
Alternative types of catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403. These patents disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene. As disclosed, for example, in the aforementioned U.S. Pat. No. 4,794,096, stereorigidity in a metallocene ligand is imparted by means of a structural bridge extending between cyclopentadienyl groups. Specifically disclosed in this patent are stereoregular hafnium metallocenes which may be characterized by the following formula: EQU R"(C.sub.5 (R').sub.4).sub.2 HfQp (3)
In formula (3), (C.sub.5 (R').sub.4) is a cyclopentadienyl or substituted cyclopentadienyl group, R' is independently hydrogen or a hydrocarbyl radical having 1-20 carbon atoms, and R" is a structural bridge extending between the cyclopentadienyl rings. Q is a halogen or a hydrocarbon radical, such as an alkyl, aryl, alkenyl, alkylaryl, or arylalkyl, having 1-20 carbon atoms and p is 2.
The various metallocene structures as described above can be used either as so-called "neutral metallocenes" in which case an alumoxane, such as methylalumoxane, is used as a co-catalyst, or they can be employed as so-called "cationic metallocenes" which incorporate a stable non-coordinating anion and normally do not require the use of an alumoxane. For example, syndiospecific cationic metallocenes are disclosed in U.S. Pat. No. 5,243,002 to Razavi. As disclosed there, the metallocene cation is characterized by the cationic metallocene ligand having sterically dissimilar ring structures which are joined to a positively-charged coordinating transition metal atom. The metallocene cation is associated with a stable non-coordinating counter-anion. Similar relationships can be established for isospecific metallocenes.
While metallocene catalysts are generally proposed for use as homogeneous catalysts, it is also known in the art to provide supported metallocene catalysts. As disclosed in U.S. Pat. Nos. 4,701,432 and 4,808,561, both to Welborn, a metallocene catalyst component may be employed in the form of a supported catalyst. As described in the Welborn '432 patent, the support may be any support such as talc, an inorganic oxide, or a resinous support material such as a polyolefin. Specific inorganic oxides include silica and alumina, used alone or in combination with other inorganic oxides such as magnesia, zirconia and the like. Non-metallocene transition metal compounds, such as titanium tetrachloride, are also incorporated into the supported catalyst component. The Welborn '561 patent discloses a heterogeneous catalyst which is formed by the reaction of a metallocene and an alumoxane in combination with the support material. A catalyst system embodying both a homogeneous metallocene component and a heterogeneous component, which may be a "conventional" supported Ziegler-Natta catalyst, e.g. a supported titanium tetrachloride, is disclosed in U.S. Pat. No. 5,242,876 to Shamsoum et al. Various other catalyst systems involving supported metallocene catalysts are disclosed in U.S. Pat. Nos. 5,308,811 to Suga et al and 5,444,134 to Matsumoto.
Polyolefin compositions which can be oriented at variable process conditions, particularly over a range of machine direction orientation draw ratios and transverse direction orientation oven temperatures, are desirable for a number of reasons. Film manufacturers have the flexibility to vary one or more processing conditions within an acceptable range for a particular film production run. In addition, the possibility of a web break during the orientation process is lessened, resulting in lower processing line start up costs. For example, a standard machine direction orientation draw ratio is about 5 times the original length of the polyolefin film. A standard transverse direction oven temperature is about 166.degree. C. These processing conditions are considered standard in that the occurrence of web breaks in the film are infrequent. The ability to orient film compositions under variable conditions, for example a machine direction draw ratio within the range of its standard up to 9 times the original length, and a transverse direction oven temperature within the range of its standard to about 20.degree. C. less than the standard, gives manufacturers greater latitude in the production of film products. A further processing advantage is the ability to draw the film at higher line speeds thereby decreasing the manufacturing time.
Biaxially-oriented films can have a number of properties to their advantage during and after the machine processing steps. A relatively low coefficient friction is desirable, both during the biaxially orientation procedure and in the use of the ultimately-produced biaxially-oriented film for end use applications. A relatively high stiffness, as indicated by the tensile modulus in both the machine direction and the transverse direction is usually advantageous. Relatively low permeabilities to gas and water are desirable. In addition, a high shrinkage factor of the processed film, while undesirable in some cases, can be advantageous in others, such as where the film is used in stretch wrapping of food products, electrical components, and the like.
Properties of the resulting film product can be dependent to a certain degree on the particular process conditions under which the polyolefin composition was manufactured. For example, a stiffer film with a higher shrinkage factor and better barrier properties would result from an orientation process incorporating a larger machine direction orientation draw ratio. Likewise, the transverse direction orientation oven temperature would affect the properties of the resulting oriented film product, particularly improving the shrinkage factor.
The physical and optical properties of films are important in the film industry and should fall within certain parameters for different film applications. The optical properties include haze, contact clarity (NAS), and gloss. Haze is a phenomena of light scattering and arises from local variations in the refractive index. Haze is defined as the relative fraction of scattered intensity from the dispersed particles in all directions, being detected in a range of wide angle, to the incident light intensity. Contact clarity or NAS is a measure of contact clearness or see-through quality and is different from haze due to the direct transmittance of light. For example, some films may indeed be hazy but appear clear as the film is in contact with the contents of a package. Unlike haze, NAS clarity is distance dependent so that the thinner the film, the better the contact clarity. Gloss is defined as the ratio of the reflected light intensity from the film at a specific angle of incidence light to that of a standard with the ideal smooth surface.